Age-related tolerance to osmotic stress: implications for reproduction and recruitment ecology of Mesodesma donacium in the Humboldt Current Ecosystem

The surf clam, Mesodesma donacium, is an ecologically and economically important species in sandy beach ecosystems of Chile and Peru. The massive mortality of this clam after strong El Niño events causes a significant reduction of the northern edge of geographic distribution, revealing a low ability to recover that might be related to reproduction and recruitment constraints. From an integrative data analysis comprising eight independent studies (including our own two-year data) revering to seven sandy beaches covering the entire geographic distribution, we determined the spatial-temporal patterns of the reproductive cycle and evaluated potential environmental factors explaining the observed patterns by using a multivariate analysis. During a one-month laboratory study we determined the effect of temperature (17.5, 23.5º C) and salinity(10, 20, 35 psu) on body condition, growth and mortality of this clam.The reproductive cycle of M. donacium showed a marked geographic pattern. Northern populations (16 - 32º S) showed an austral-spring spawning and southern (42 - 43º S) populations showed an austral-autumn spawning, with transitional populations in Mehuin (39º S), which exhibit a biannual, spring and autumn spawning seasons. Sea surface temperature variability showed a consistent seasonal latitudinal pattern, which could only explain the northern pattern of the reproductive cycle. In contrast, fresh-water input (river inflow and coastal precipitation) seems to explain better the observed pattern, as seasonality was not the same: pluvial, mixed and nival river regimes exhibit different seasonal patterns along latitude.A preliminary analysis showed that performance of M. donacium was not affected at low salinity (20 psu) but is negatively affected by extreme low salinity (10 psu) and anomalous high temperature (23.5º C). When analyzed by age groups, juveniles showed faster growth rates, higher body condition and lower mortality than adults at low salinities (10, 20 psu). They also seem to perform better than adults at anomalous high temperature. Although we used clams from the only place having apparently no influence of fresh-water input, M. donacium (notably juveniles) seems to be tolerant to osmotic stress. We discuss this finding and the fact that six out of seven of the studied populations are directly influenced by river inflow, as an adaptation of this species, to find suitable areas for settlement and recruitment. This is supported by previous observations showing that i) interspecific interactions play a critical role on the recruitment ecology of Mesodesma spp and Donax spp (species with a similar ecological role in sandy beaches); ii) M. donacium shows a definite spatial distribution pattern with age classes segregation along gradients of distance to river mouths and iii) former populations disappeared after closings of river input to the coastal zone. The availability of suitable places for settlement and recruitment may play a critical role in explaining the recovery ability of M. donacium after El Niñoty given the distribution of rivers flowing to the coast in Chile and Peru, with the Atacama dessert region acting as a geographic barrier to the northward distribution of this species